The invention generally relates to liquid gauging systems, and it more particularly pertains to method and apparatus for measuring the density, volume and/or mass of fuel contained in a partially filled tank such as an aircraft aviation fuel tank.
While the invention disclosed hereinafter has general application to the technology of liquid gauging, the preferred embodiment of the invention concerns the problems and requirements that are present in measuring fuel contained in aircraft tanks. Aviation fuel tanks are of varied and complex shape and are generally mounted in the wings and fuselage with the shape of the tank being dictated by the profile of the aircraft structure. The irregular geometry of the tanks and the different attitudes that can be assumed by the aircraft at any given instant that the fuel level is monitored, are factors that contribute to the difficulty of accurately measuring the remaining quantity of fuel. Additionally, the aircraft tanks are routinely filled from a variety of aviation fuel sources, and the density of fuel from these different sources can and typically does vary widely. Since the fuel density is related to the energy content in any given unit volume of fuel, it is important to know not only the volume of the remaining fuel but also its density and hence mass or energy content, thus adding an additional design complication to the developement of suitable gauging instrumentation.
One of the more common fuel metering systems uses a plurality of electrical capacitance sensors. Each such sensor comprises a cylindrical open pipe disposed vertically in the tank and surrounding a coaxial center conductor. An AC electrical signal is applied across the outer pipe and center conductor. The fuel fills the space between the inner wall of the cylindrical pipe and the center conductor to the surrounding level of fuel in the tank, thus providing a different (larger) dielectric constant for the portion of the sensor that is submerged below the fuel level compared to the unsubmerged portion, where the dielectric constant of air causes a lower effective dielectric. The plurality of sensors in a given tank are wired in parallel to sum the overall capacitances and this sum is then correlated to the total fuel content. Usually the capacitance sensors are tailored to a particular tank geometry by shaping the inner center conductor, although such shaping does not always precisely match the sensor output to the content of the remaining fuel. Furthermore, such capacitance sensors are usually not augmented with devices for measuring the density of the fuel and hence deviations in the mass (energy content) of the fuel due to variations in density are not measured. Rather, an average fuel density is assumed or estimated and that value is used to compute the mass of the fuel.
The capacitance-type sensors are frequently impaired by corrosion, microbe growth and other contaminants present in the fuel which introduce deviations in the dielectric constant and hence in the measurement performed by the sensor. Electromagnetic interference may also cause the capacitance sensors to deviate from a true fuel content measurement. Different pitch and roll attitudes of the aircraft introduce variations between the actual remaining fuel content and the amount of fuel as measured by capacitance sensors and the relations between various attitudes and fuel measurements are not readily correlated and accounted for in the instrumentation. These factors, which contribute to imprecise fuel content measurements, are compensated for by carrying additional fuel in each of the tanks using worst-case assumptions, sufficient to more than compensate for tolerances in the fuel measurement instrumentation based on capacitance sensors.